1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device and more particularly to a threshold voltage read method which reads out a threshold voltage of a rewritable nonvolatile semiconductor memory device and a nonvolatile semiconductor memory device used to perform the read method.
2. Description of the Related Art
Recently, the memory capacity of a rewritable nonvolatile semiconductor memory device, for example, the memory capacity of a NAND flash memory has been stably increased. As the technique for increasing the memory capacity, the technique for miniaturizing memory cells and forming memory data into a multi-level form is provided. The distribution of threshold voltages of memory cells of a multi-level flash memory is narrower than that of a binary memory and the control operation for the threshold voltages is strict than that in the binary memory. Further, since a plurality of read voltages applied to word lines are required, it is necessary to precisely perform the strict threshold voltage control operation.
In order to precisely perform the threshold voltage control operation, for example, it is necessary to make a highly precise circuit design of a data write circuit such as a page buffer. In order to make the highly precise circuit design, it is preferable to acquire actual information from the device itself and feed back the actual information to the circuit design.
As one example of the operation of acquiring actual information, an item of “measurement of distribution of threshold voltages of memory cells” is provided. The item is to actually read out and check the form of the threshold voltage distribution from the device itself after data is actually written into a memory cell. The information acquired at this time is utilized for determining various control parameters for controlling the threshold voltages, such as timing setting values used for write verify operations or read operations, and voltages applied to word and bit lines.
However, when the accuracy of the read threshold voltage is not so high, or when there is no excellent function of estimating the threshold voltage distribution, it adversely affects the optimization of the flash memory that stores multi-level data.